1. Field of the Invention
The invention relates to a device for forming a an image bounded by a circle of radius R on a display screen, by scanning the display screen in a pattern of parallel image lines and for forming an output signal at any instant at which an image line intersects the circumference of the circle to be formed.
2. Description of the Related Art
A device of this kind may form part of, for example an X-ray apparatus comprising an image intensifier, a television camera for picking up the image formed on the exit screen of the image intensifier, and a monitor for displaying the image picked up by the camera, for example as described in GB-A-2 035 749. Because the exit screen of the image intensifier usually has a circular shape and the display screen of the monitor has a rectangular shape, it is desirable to darken the part of the display screen outside the image of the exit screen, so that the observation of this image, containing the desired information, is not impeded by disturbing light. This can be readily realised by forming, on the display screen, an image of a black surface with a circular opening having the same dimensions as the exit screen, said surface being positioned so that the image of the exit screen is coincident with the opening in the surface. Moreover, it is often desirable to indicate a circular measuring region in an object displayed on the display screen, for which purpose a circular image is superposed on the image of the object. This can be carried out during X-ray examinations as well as during various other measurements (for example, ultrasound examinations for medical diagnosis or testing of materials). In all cases it is desirable to generate a circular image which can be added to the actual image displayed on the display screen. This image was generated thus far using analog means. This operation requires a comparatively large amount of time, which is disturbing notably when frequently a new circle (for example, having a different radius or a different position) is to be generated.